Trap engineering in solution processed PbSe quantum dots for high-speed MID-infrared photodetectors

Abstract

The ongoing quest to find methods to control the trap states in solution processed nanostructures (trap engineering) will revolutionise the applications of nanomaterials for optoelectronic purposes. In this paper, we present a new combined experimental/theoretical approach (molecular orbital theory) allowing a new view on trap engineering of nanostructures for applications in photodetectors. PbSe quantum dots (QDs) of about 30 nm diameter were prepared in a solution-based process from lead iodide (PbI₂) and chloride (PbCl₂), while using lead acetate (PbOAc₂) reliably gave particles of about 200 nm in size under the same conditions. Comparison of the dangling acetate (OAc⁻) versus the spherical monoatomic surface ligands chloride (Cl⁻) and iodide (I⁻) and varying the covalent/ionic character of the particle-surface ligand (ionic: OAc⁻ > Cl⁻ > I⁻: covalent) bond allowed an interesting insight into what governs the trap states. Density functional theory (DFT) calculations are used to study band structures and density of states and show trap states localised within the bandgap moving to the conduction or valence band upon interaction of surface metal atoms with the surface ligands. Infrared detectors based on these materials are fabricated and allowed high-speed mid-infrared photo-detection with 100 ns rise and 110 ns fall response times.